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Synthesis of calcium hexaaluminate (CaAl12O19) via reverse micelle process
LETTER TO THE EDITOR
Journal of Non-Crystalline Solids 355 (2009) 2429–2432
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Synthesis of calcium hexaaluminate (CaAl12O19) via reverse micelle process J. Chandradass a,*, Dong Sik Bae b, Ki Hyeon Kim a,* a b
Department of Physics, Yeungnam University, Gyeongsan, Gyeongsangbuk-do 712-749, South Korea School of Nano & Advanced Materials Engineering, Changwon National University, Changwon 641-773, Korea
a r t i c l e
i n f o
Article history: Received 8 July 2009 Available online 26 September 2009 PACS: 81.05 81.16 82.70 68.37
Je Be Uv Hk
a b s t r a c t This letter describes the synthesis of CaAl12O19 powders using micro reactors made of Igepal CO520/ water/cyclohexane microemulsions. Characterization of the powder was done by DTA-TGA, X-ray diffraction, Scanning electron microscopy, Fourier Transform Infrared Spectroscopy. The XRD results show that the hexagonal CaAl12O19 powders have been obtained at 1200 °C for 2 h. The SEM examination shows that the hexagonal CaAl12O19 has plate-like grain morphology with most of the grain took the form of hexagonal platelets with-developed faces. The FTIR spectra show the lower frequency bands are assigned to AlO6 octohedra and AlO4 tetrahedra in CaAl12O19. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Ceramics Scanning electron microscopy Powders X-ray diffraction
1. Introduction Calcium hexaluminate, CaAl12O19 (hibonite) has the magnetoplumbite crystal structure [1] in which the calcium occurs in cleavage planes located between blocks of Al2O3 spinel structure containing 20 octahedral Al and two tetrahedral Al per unit cell. The remaining two Al atoms per unit cell are associated with the cleavage plane, and are suggested to be in irregular five-coordinated environments. The behavior of Al in this phase during its structural evolution is therefore of interest for the unusual Al environments in both amorphous and crystalline aluminates. Calcium hexaluminate, CaAl12O19, has shown promise as an interface phase in fiber-reinforced ceramic oxide composites because of its preferred basal-plane cleavage properties [2]. Calcium hexaluminate, CaAl12O19, is an oxide-based alternative to carbon and boron nitride for fiber coating in ceramic–matrix composites in oxidizing environments at temperatures > 1000 °C, but its synthesis as single phase powder requires high temperature treatments [3]. MacKenzie et al. [4] prepared calcium hexaaluminate by Pechini method and shows magnetoplumbite structure formed in an exothermic reaction in which the large Ca ion migrates into the mirror planes between the spinel blocks. Singh et al. [5] prepared Cr3+ activated CaAl12O19 phosphors by combustion process. Evidence for the exis-
tence of Cr3+ ions in the doped lattice is presented. Costa et al. [6] prepared a new turquoise blue ceramic pigment on the basis of nickel-doped hibonite. Nie et al. [7] reported the Cr3+ ion as a codopant to modify the unpractical Photon cascade emission properties of Pr3+ in CaAl12O19 phosphors. Among all the chemical processes that were developed for the preparation of fine ceramic powders and for producing a wide array of metals and metal oxide compounds [8–10] the microemulsion processing involving reverse micelles has been demonstrated as a superior method [11] in terms of being able to deliver homogeneous and nanosized grains of a variety of oxides. This aqueous method uses readily available inexpensive, and easily handled precursors of Ca(NO3)3 and Al(NO3)3, and eliminates the extra handling requirements that usually associates with moisture sensitive precursors. In recent years, microemulsion method has been studied widely and has been key technique to synthesize oxide nanoparticles owing to the products which has a characteristic of well dispersed, controlled size and narrow size distribution. However, there is no literature available on the synthesis of Calcium hexaaluminate (CaAl12O19) by reverse micelle process. In the present work, we prepared pure CaAl12O19 for the first time based on Igepal CO520/water/cyclohexane reverse microemulsion route at 1200 °C for 2 h. 2. Experimental
* Corresponding authors. Tel.: +82 53 810 2334; fax: +82 53 810 4616. E-mail addresses: [email protected] (J. Chandradass), [email protected] (K.H. Kim). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.08.032
Fig. 1 shows the flow chart for the preparation of CaAl12O19 by reverse micelle process. Calcium nitrate (Ca(NO3)24H2O),
LETTER TO THE EDITOR
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J. Chandradass et al. / Journal of Non-Crystalline Solids 355 (2009) 2429–2432
Fig. 1. Flow chart for the preparation of CaAl12O19 powders by reverse micelle processing.
aluminum nitrate (Al(NO3)39H2O were used as the precursor of calcia and alumina. Cyclohexane (Sigma Aldrich) was used as organic solvent. Reverse microemulsion solution was prepared by mixing 40 mL of nonionic surfactant poly(oxyethylene) nonylphenyl ether (Igepal CO-520, Aldrich Chemical Co., USA), 100 mL of cyclohexane and 13.2 mL of mixed aqueous solution (Ca:Al = 1:2). The water/surfactant (R) was maintained at 8. The microemulsion was mixed rapidly, and after 5 min of equilibration, 6 ml of NH4OH (28%) (Dae Jung chemicals, Korea) was injected into the microemulsion. The microemulsion was then centrifuged to extract the particles, which were subsequently washed by ethanol to remove any residual surfactant. The phase identification of calcined powders was recorded by X-ray diffractometer (Philips X’pert MPD 3040). The morphology of the calcined powder was observed by Scanning electron microscopy (SEM) operating at an accelerating voltage of 30 kV. The Fourier transform infrared spectra (FTIR)
were measured on a Nicolet Impact 410 DSP spectrophotometer using the KBr pellet method.
3. Results and discussion Thermal behavior of the precursor powders determined by DTA/ TG in oxygen atmosphere up to 1400 °C at a heating rate of 10 °C/ min. is shown in Fig. 2. Decomposition started below 150 °C with a weight loss of 5.35% corresponds to adsorbed water. In the temperature region between 150–600 °C, the main decomposition occurs with a weight loss of 40.7%. The thermal decomposition behavior is associated with endothermic and exothermic effects in the DTA curve. It may be inferred that endothermic peak between 200– 300 °C represents the decomposition of Al(OH)3. Previous workers [12–14] have reported Al(OH)3 decomposition takes place at 300,
Fig. 2. DTA and TGA studies of as-synthesized precursor.
LETTER TO THE EDITOR
J. Chandradass et al. / Journal of Non-Crystalline Solids 355 (2009) 2429–2432
375, and 425 °C. Exothermic peak in the temperature range 300– 600 °C in the DTA curve is due to the burning of residual surfactant. The final weight loss (1.8%) on the TG curve was in the temperature > 600 °C is mainly due to the desorption of residual hydroxyl (OH) group [15]. The DTA curve did not show the exothermic peak related to crystallization of CaAl12O19. Fig. 3 shows the XRD patterns of the CaAl12O19 precursor with R = 8 and calcined in static air at 800–1200 °C for 2 h. After heating the precursor at 800 °C in air for 2 h, the powder is amorphous. Increasing the calcinations temperature to 1000 °C, the crystallinity slightly increases and c-Al2O3 phase has been identified. With the temperature increasing from 1000 °C to 1100 °C, c-Al2O3 phase disappears and CaAl12O19 start to crystallize. At 1200 °C, the diffraction peaks from CaAl12O19 become sharper indicating the growth of the particles. All of the detectable peaks can be indexed as the hexagonal structure in the standard data (ICSD: 01-0841613) for CaAl12O19 samples calcined at 1200 °C, respectively. This synthesis method produces a crystalline monophasic product with no evidence of intermediates or secondary phases, by contrast with the result of Cinibulk [3] who reported the formation of small amount of intermediate CaAl4O7 prepared by Pechini method. MacKenzie et al. [4] reported the formation of calcium hexaaluminate by Pechini method at 1400 °C. In the present study we were able to synthesis calcium hexaaluminate at 1200 °C. This is a promising result compared with those reported in the literature. Surface features of CaAl12O19 samples calcined at 1200 °C are studied using SEM (Fig. 4). Previous workers [16,17] have reported plate-like grain morphologies are obtained for CaAl12O19 synthesized by reaction sintering and equiaxed grains are obtained when it is produced by milling and sintering. Recently Singh et al. [18] showed plate-like morphology for Cr:CaAl12O19 powders prepared by combustion method, which involves reaction sintering and no post-sintering treatments. In accordance with this report, our powders also remain mostly the same and consist of faceted plates with most of the calcium hexaaluminates took the form of hexagonal platelets with-developed faces as reported elsewhere [19]. Fig. 5 shows the FTIR analysis of microemulsion-derived precursor and calcined powder at R = 8. The presence of these organic residuals in the as-dried precursor is indicated by the sharp bands over the wavenumber range of 1300–1400 and 1600–1700 cm 1, which corresponds to C–O and C = C stretching vibrations, respectively. These functional groups were not completely eliminated
2431
Fig. 4. SEM micrographs of as-synthesized precursor calcined at1200 °C.
Fig. 5. FTIR analysis of as-synthesized precursor (a) dried and (b) 1200 °C.
when the precursor was repeatedly washed and subsequently dried at 80 °C. As expected, the hydroxide nature of the as-dried precursor was indicated by a relatively broad vibrational band over the range of 3200–3700 cm 1. The absorption band in the region 2370 cm 1 indicates the presence of CO2 absorption. The band intensities related to hydroxyl group and CO2 band is still visible in the calcined sample. This may be due to the moisture absorption during testing. The presence of organics in the calcined sample, which provides a unique and desirable situation for limiting the growth of the particle size [20]. The visible band over the range of 1000–400 cm 1 corresponds to metal–oxygen bonds (possible Al–O stretching frequencies). The band between 900–700 cm 1 and 700–200 cm 1 corresponds to AlO6 and AlO4 vibrations, respectively [21]. 4. Conclusion
Fig. 3. XRD patterns of as-synthesized precursor calcined at different temperature (a) 800 °C; (b) 1000 °C; (c) 1100 °C; and (d) 1200 °C.
CaAl12O19 was successfully synthesized using reverse micelle process. The XRD results show that the hexagonal CaAl12O19 powders have been obtained at 1200 °C for 2 h. The SEM examination
LETTER TO THE EDITOR
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J. Chandradass et al. / Journal of Non-Crystalline Solids 355 (2009) 2429–2432
shows that the hexagonal CaAl12O19 has plate-like grain morphology with most of the grain took the form of hexagonal platelets with-developed faces. The FTIR spectra show the lower frequency bands are assigned to AlO6 octohedra and AlO4 tetrahedra in CaAl12O19. References [1] A. Utsunomiya, K. Tanaka, H. Morikawa, F. Marumo, H. Kojima, J. Solid State Chem. 75 (1988) 197. [2] M.K. Cinibulk, Ceram. Eng. Sci. Proc. 16 (1995) 633. [3] M.K. Cinibulk, J. Am. Ceram. Soc. 81 (1998) 3157. [4] K.J.D. MacKenziea, M. SchmuÈcker, M.E. Smith, I.J.F. Poplett, T. Kemmitt, Thermochim. Acta 363 (2000) 181. [5] Vijay Singh, R.P.S. Chakradhar, J.L. Rao, Dong-Kuk Kim, Solid State Sci. 10 (2008) 1525. [6] G. Costa, M.J. Ribeiro, W. Hajjaji, M.P. Seabra, J.A. Labrincha, M. Dondi, G. Cruciani, J. Eur. Ceram. Soc. (2009), doi:10.1016/j.jeurceramsoc.2009.04.001.
[7] Z.G. Nie, J.H. Zhang, X. Zhanga, S.Z. Lu, X.G. Rena, G.B. Zhang, X.J. Wanga, J. Solid State Chem. 180 (2007) 2933. [8] T. Sugimoto, K. Kimijima, J. Phys. Chem. B 107 (2003) 10753. [9] M.C. McLeod, R.S. McHenry, E.J. Beckman, C.B. Roberts, J. Phys. Chem. B 107 (2003) 2693. [10] W. Sun, L. Xu, Y. Chu, W. Shi, J. Colloid Interface Sci. 266 (2003) 99. [11] M. Boutonnet, J. Kizling, P. Stenius, G. Maire, Colloids Surf. 5 (1982) 209. [12] S.K. Behera, P.K. Sahu, S.K. Pratihar, S. Bhattacharya, Mater. Lett. 58 (2004) 3710. [13] S. Bhattacharya, S.K. Pratihar, R.K. Sinha, R.C. Behera, R.I. Ganguly, Mater.Lett. 53 (2002) 425. [14] Z. Tian, W. Huang, Y. Liang, Ceram. Int. 35 (2009) 661. [15] N. Yoshio, K. Mitsuo, N. Junichi, J. Phys. Chem. B 102 (1998) 10279. [16] L.C. De Jonghe, H. Schmid, M. Chang, J. Am. Ceram. Soc. 67 (1984) 27. [17] L. An, H.M. Chan, K.K. Soni, J. Mater. Sci. 31 (1996) 3223. [18] Vijay Singh, R.P.S. Chakradhar, J.L. Rao, Dong-Kuk Kim, Solid State Sci. 10 (2008) 1525. [19] Vijay Singh, T.K. Gundu Rao, Jun-Jie Zhu, J. Lumin. 126 (2007) 1. [20] C.-P. Lin, S-B. Wen, T.-T. Lee, J. Am. Ceram. Soc. 85 (2002) 129. [21] G.D. Chukin, Yu.L. Seleznev, Kinet. Katal. 30 (1989) 55.
Journal of Non-Crystalline Solids 355 (2009) 2429–2432
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Synthesis of calcium hexaaluminate (CaAl12O19) via reverse micelle process J. Chandradass a,*, Dong Sik Bae b, Ki Hyeon Kim a,* a b
Department of Physics, Yeungnam University, Gyeongsan, Gyeongsangbuk-do 712-749, South Korea School of Nano & Advanced Materials Engineering, Changwon National University, Changwon 641-773, Korea
a r t i c l e
i n f o
Article history: Received 8 July 2009 Available online 26 September 2009 PACS: 81.05 81.16 82.70 68.37
Je Be Uv Hk
a b s t r a c t This letter describes the synthesis of CaAl12O19 powders using micro reactors made of Igepal CO520/ water/cyclohexane microemulsions. Characterization of the powder was done by DTA-TGA, X-ray diffraction, Scanning electron microscopy, Fourier Transform Infrared Spectroscopy. The XRD results show that the hexagonal CaAl12O19 powders have been obtained at 1200 °C for 2 h. The SEM examination shows that the hexagonal CaAl12O19 has plate-like grain morphology with most of the grain took the form of hexagonal platelets with-developed faces. The FTIR spectra show the lower frequency bands are assigned to AlO6 octohedra and AlO4 tetrahedra in CaAl12O19. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Ceramics Scanning electron microscopy Powders X-ray diffraction
1. Introduction Calcium hexaluminate, CaAl12O19 (hibonite) has the magnetoplumbite crystal structure [1] in which the calcium occurs in cleavage planes located between blocks of Al2O3 spinel structure containing 20 octahedral Al and two tetrahedral Al per unit cell. The remaining two Al atoms per unit cell are associated with the cleavage plane, and are suggested to be in irregular five-coordinated environments. The behavior of Al in this phase during its structural evolution is therefore of interest for the unusual Al environments in both amorphous and crystalline aluminates. Calcium hexaluminate, CaAl12O19, has shown promise as an interface phase in fiber-reinforced ceramic oxide composites because of its preferred basal-plane cleavage properties [2]. Calcium hexaluminate, CaAl12O19, is an oxide-based alternative to carbon and boron nitride for fiber coating in ceramic–matrix composites in oxidizing environments at temperatures > 1000 °C, but its synthesis as single phase powder requires high temperature treatments [3]. MacKenzie et al. [4] prepared calcium hexaaluminate by Pechini method and shows magnetoplumbite structure formed in an exothermic reaction in which the large Ca ion migrates into the mirror planes between the spinel blocks. Singh et al. [5] prepared Cr3+ activated CaAl12O19 phosphors by combustion process. Evidence for the exis-
tence of Cr3+ ions in the doped lattice is presented. Costa et al. [6] prepared a new turquoise blue ceramic pigment on the basis of nickel-doped hibonite. Nie et al. [7] reported the Cr3+ ion as a codopant to modify the unpractical Photon cascade emission properties of Pr3+ in CaAl12O19 phosphors. Among all the chemical processes that were developed for the preparation of fine ceramic powders and for producing a wide array of metals and metal oxide compounds [8–10] the microemulsion processing involving reverse micelles has been demonstrated as a superior method [11] in terms of being able to deliver homogeneous and nanosized grains of a variety of oxides. This aqueous method uses readily available inexpensive, and easily handled precursors of Ca(NO3)3 and Al(NO3)3, and eliminates the extra handling requirements that usually associates with moisture sensitive precursors. In recent years, microemulsion method has been studied widely and has been key technique to synthesize oxide nanoparticles owing to the products which has a characteristic of well dispersed, controlled size and narrow size distribution. However, there is no literature available on the synthesis of Calcium hexaaluminate (CaAl12O19) by reverse micelle process. In the present work, we prepared pure CaAl12O19 for the first time based on Igepal CO520/water/cyclohexane reverse microemulsion route at 1200 °C for 2 h. 2. Experimental
* Corresponding authors. Tel.: +82 53 810 2334; fax: +82 53 810 4616. E-mail addresses: [email protected] (J. Chandradass), [email protected] (K.H. Kim). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.08.032
Fig. 1 shows the flow chart for the preparation of CaAl12O19 by reverse micelle process. Calcium nitrate (Ca(NO3)24H2O),
LETTER TO THE EDITOR
2430
J. Chandradass et al. / Journal of Non-Crystalline Solids 355 (2009) 2429–2432
Fig. 1. Flow chart for the preparation of CaAl12O19 powders by reverse micelle processing.
aluminum nitrate (Al(NO3)39H2O were used as the precursor of calcia and alumina. Cyclohexane (Sigma Aldrich) was used as organic solvent. Reverse microemulsion solution was prepared by mixing 40 mL of nonionic surfactant poly(oxyethylene) nonylphenyl ether (Igepal CO-520, Aldrich Chemical Co., USA), 100 mL of cyclohexane and 13.2 mL of mixed aqueous solution (Ca:Al = 1:2). The water/surfactant (R) was maintained at 8. The microemulsion was mixed rapidly, and after 5 min of equilibration, 6 ml of NH4OH (28%) (Dae Jung chemicals, Korea) was injected into the microemulsion. The microemulsion was then centrifuged to extract the particles, which were subsequently washed by ethanol to remove any residual surfactant. The phase identification of calcined powders was recorded by X-ray diffractometer (Philips X’pert MPD 3040). The morphology of the calcined powder was observed by Scanning electron microscopy (SEM) operating at an accelerating voltage of 30 kV. The Fourier transform infrared spectra (FTIR)
were measured on a Nicolet Impact 410 DSP spectrophotometer using the KBr pellet method.
3. Results and discussion Thermal behavior of the precursor powders determined by DTA/ TG in oxygen atmosphere up to 1400 °C at a heating rate of 10 °C/ min. is shown in Fig. 2. Decomposition started below 150 °C with a weight loss of 5.35% corresponds to adsorbed water. In the temperature region between 150–600 °C, the main decomposition occurs with a weight loss of 40.7%. The thermal decomposition behavior is associated with endothermic and exothermic effects in the DTA curve. It may be inferred that endothermic peak between 200– 300 °C represents the decomposition of Al(OH)3. Previous workers [12–14] have reported Al(OH)3 decomposition takes place at 300,
Fig. 2. DTA and TGA studies of as-synthesized precursor.
LETTER TO THE EDITOR
J. Chandradass et al. / Journal of Non-Crystalline Solids 355 (2009) 2429–2432
375, and 425 °C. Exothermic peak in the temperature range 300– 600 °C in the DTA curve is due to the burning of residual surfactant. The final weight loss (1.8%) on the TG curve was in the temperature > 600 °C is mainly due to the desorption of residual hydroxyl (OH) group [15]. The DTA curve did not show the exothermic peak related to crystallization of CaAl12O19. Fig. 3 shows the XRD patterns of the CaAl12O19 precursor with R = 8 and calcined in static air at 800–1200 °C for 2 h. After heating the precursor at 800 °C in air for 2 h, the powder is amorphous. Increasing the calcinations temperature to 1000 °C, the crystallinity slightly increases and c-Al2O3 phase has been identified. With the temperature increasing from 1000 °C to 1100 °C, c-Al2O3 phase disappears and CaAl12O19 start to crystallize. At 1200 °C, the diffraction peaks from CaAl12O19 become sharper indicating the growth of the particles. All of the detectable peaks can be indexed as the hexagonal structure in the standard data (ICSD: 01-0841613) for CaAl12O19 samples calcined at 1200 °C, respectively. This synthesis method produces a crystalline monophasic product with no evidence of intermediates or secondary phases, by contrast with the result of Cinibulk [3] who reported the formation of small amount of intermediate CaAl4O7 prepared by Pechini method. MacKenzie et al. [4] reported the formation of calcium hexaaluminate by Pechini method at 1400 °C. In the present study we were able to synthesis calcium hexaaluminate at 1200 °C. This is a promising result compared with those reported in the literature. Surface features of CaAl12O19 samples calcined at 1200 °C are studied using SEM (Fig. 4). Previous workers [16,17] have reported plate-like grain morphologies are obtained for CaAl12O19 synthesized by reaction sintering and equiaxed grains are obtained when it is produced by milling and sintering. Recently Singh et al. [18] showed plate-like morphology for Cr:CaAl12O19 powders prepared by combustion method, which involves reaction sintering and no post-sintering treatments. In accordance with this report, our powders also remain mostly the same and consist of faceted plates with most of the calcium hexaaluminates took the form of hexagonal platelets with-developed faces as reported elsewhere [19]. Fig. 5 shows the FTIR analysis of microemulsion-derived precursor and calcined powder at R = 8. The presence of these organic residuals in the as-dried precursor is indicated by the sharp bands over the wavenumber range of 1300–1400 and 1600–1700 cm 1, which corresponds to C–O and C = C stretching vibrations, respectively. These functional groups were not completely eliminated
2431
Fig. 4. SEM micrographs of as-synthesized precursor calcined at1200 °C.
Fig. 5. FTIR analysis of as-synthesized precursor (a) dried and (b) 1200 °C.
when the precursor was repeatedly washed and subsequently dried at 80 °C. As expected, the hydroxide nature of the as-dried precursor was indicated by a relatively broad vibrational band over the range of 3200–3700 cm 1. The absorption band in the region 2370 cm 1 indicates the presence of CO2 absorption. The band intensities related to hydroxyl group and CO2 band is still visible in the calcined sample. This may be due to the moisture absorption during testing. The presence of organics in the calcined sample, which provides a unique and desirable situation for limiting the growth of the particle size [20]. The visible band over the range of 1000–400 cm 1 corresponds to metal–oxygen bonds (possible Al–O stretching frequencies). The band between 900–700 cm 1 and 700–200 cm 1 corresponds to AlO6 and AlO4 vibrations, respectively [21]. 4. Conclusion
Fig. 3. XRD patterns of as-synthesized precursor calcined at different temperature (a) 800 °C; (b) 1000 °C; (c) 1100 °C; and (d) 1200 °C.
CaAl12O19 was successfully synthesized using reverse micelle process. The XRD results show that the hexagonal CaAl12O19 powders have been obtained at 1200 °C for 2 h. The SEM examination
LETTER TO THE EDITOR
2432
J. Chandradass et al. / Journal of Non-Crystalline Solids 355 (2009) 2429–2432
shows that the hexagonal CaAl12O19 has plate-like grain morphology with most of the grain took the form of hexagonal platelets with-developed faces. The FTIR spectra show the lower frequency bands are assigned to AlO6 octohedra and AlO4 tetrahedra in CaAl12O19. References [1] A. Utsunomiya, K. Tanaka, H. Morikawa, F. Marumo, H. Kojima, J. Solid State Chem. 75 (1988) 197. [2] M.K. Cinibulk, Ceram. Eng. Sci. Proc. 16 (1995) 633. [3] M.K. Cinibulk, J. Am. Ceram. Soc. 81 (1998) 3157. [4] K.J.D. MacKenziea, M. SchmuÈcker, M.E. Smith, I.J.F. Poplett, T. Kemmitt, Thermochim. Acta 363 (2000) 181. [5] Vijay Singh, R.P.S. Chakradhar, J.L. Rao, Dong-Kuk Kim, Solid State Sci. 10 (2008) 1525. [6] G. Costa, M.J. Ribeiro, W. Hajjaji, M.P. Seabra, J.A. Labrincha, M. Dondi, G. Cruciani, J. Eur. Ceram. Soc. (2009), doi:10.1016/j.jeurceramsoc.2009.04.001.
[7] Z.G. Nie, J.H. Zhang, X. Zhanga, S.Z. Lu, X.G. Rena, G.B. Zhang, X.J. Wanga, J. Solid State Chem. 180 (2007) 2933. [8] T. Sugimoto, K. Kimijima, J. Phys. Chem. B 107 (2003) 10753. [9] M.C. McLeod, R.S. McHenry, E.J. Beckman, C.B. Roberts, J. Phys. Chem. B 107 (2003) 2693. [10] W. Sun, L. Xu, Y. Chu, W. Shi, J. Colloid Interface Sci. 266 (2003) 99. [11] M. Boutonnet, J. Kizling, P. Stenius, G. Maire, Colloids Surf. 5 (1982) 209. [12] S.K. Behera, P.K. Sahu, S.K. Pratihar, S. Bhattacharya, Mater. Lett. 58 (2004) 3710. [13] S. Bhattacharya, S.K. Pratihar, R.K. Sinha, R.C. Behera, R.I. Ganguly, Mater.Lett. 53 (2002) 425. [14] Z. Tian, W. Huang, Y. Liang, Ceram. Int. 35 (2009) 661. [15] N. Yoshio, K. Mitsuo, N. Junichi, J. Phys. Chem. B 102 (1998) 10279. [16] L.C. De Jonghe, H. Schmid, M. Chang, J. Am. Ceram. Soc. 67 (1984) 27. [17] L. An, H.M. Chan, K.K. Soni, J. Mater. Sci. 31 (1996) 3223. [18] Vijay Singh, R.P.S. Chakradhar, J.L. Rao, Dong-Kuk Kim, Solid State Sci. 10 (2008) 1525. [19] Vijay Singh, T.K. Gundu Rao, Jun-Jie Zhu, J. Lumin. 126 (2007) 1. [20] C.-P. Lin, S-B. Wen, T.-T. Lee, J. Am. Ceram. Soc. 85 (2002) 129. [21] G.D. Chukin, Yu.L. Seleznev, Kinet. Katal. 30 (1989) 55.